Stall-controlled and active stall-controlled wind turbines are designed to operate at a substantially fixed speed of rotation, ie the blade rotates at the same speed irrespective of the wind speed. At high wind speeds, the power output has to be limited so as not to overload the generator. Pitch control, active stall control or (passive) stall control may be used for this purpose. At pitch control and active stall control the blades are mounted such on the hub that they are able to turn around their longitudinal axis. The rotor blades may thus be continuously adjusted to provide the lift rendering the desired power. At (passive) stall control the blades are fixedly secured to the hub and are thus not able to turn around their longitudinal axis. The stall properties of the blade is used for limiting the aerodynamic lift and thus the power output. Stall control renders a simple and reliable adjustment of the turbine, but is encumbered by the drawback that the lift varies according to the air density. The density of air depends on the air pressure and the air temperature. Usually the air temperature is the decisive factor of these two parameters. At high temperatures and thus low air density, a stall-controlled wind turbine typically does not reach the prescribed maximum power output. Conversely, during winter, where the temperature is low, the wind turbine may overproduce power, whereby the gear box or the generator may be also overloaded. Optionally a built-in security system ensures that the turbine is stopped due to overproduction.
WO 94/04820 discloses a wind turbine with stall-controlled rotor blades, and where a temperature sensor is functionally connected to an actuator for adjusting the properties of the blade such that the blade at a given wind speed is adapted to render a higher output at an increasing temperature and a reduced output at a decreasing temperature, thus at any temperature compensating for the natural tendency of the blade to produce a higher output at colder temperature. According to the WO publication this is obtained by means of a temperature-sensitive element mounted at the leading edge of the blade for adjusting the shape thereof at temperature changes. FIGS. 2 and 3 of the WO publication show the element and the shape of the leading edge are shown at a high and a low temperature, respectively. It is not disclosed how the temperature-sensitive element operates. One drawback of the blade according to the WO publication is that the element may cause sudden stall over the entire profile and thus cause a drastic reduction of the maximum power output.
U.S. Pat. No. 3,373,930 discloses a fan, where the fan blades are formed of laminates of different material layers such that the curvature of the blades varies according to the air temperature.